Use a pry bar under the rocker shaft to push down on the rocker arm tip. You should be able to move the valve through its full travel without any binding or sticking. It would be a good idea to run a little "Sea Foam" additive in the oil for a change or two and also put some in the gasoline if you suspect there might be gum on the valve guides. I add the stuff to the fuel on both my motorcycles, and it really does improve the performance, IMHO.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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